Electronic control apparatus



Feb. 24, 1948. J. J. TURlN ETAI.

ELECTRONIC CONTROL APPARATUS Filed Jan. 16, 1947 3 SheetsSheet l JbH/VJER/N IQA 4 PH H4 N/VA mwsmoas .BY {/34 Feb. 24, 1948.

J. J. TURIN ETAL ELECTRONIC CONTROL APPARATUS Filed Jan. 16, 1,947 3 Sheets-Sheet 2 IQALPH lfAN/VA INVENTORS BY 1.504

7/! eir Atto me v Patented F eb. 24, 1948 2,436,762 ELECTRONIC CONTROL APPARATUS .lohn J. Turin assignors to lloledo, Ohio,

and Ralph Hanna, Toledo, Surface Combustion Gorporation,

a corporation of Ohio Application January 16, 1947, Serial No. 722,304 3 Claims. (Cl. 250-415) This invention relates to electronic control al paratus for converting minute electrical signals into currents suitable for controlling relays. The apparatus is of the type employing electronic amplification of a type suitable for use with di-= rect current or slowly changing currents. The general object of the invention is to provide such an apparatus that is highly sensitive hut which, having, for example, responded to an increase in input signal, requires a substantial decrease in signal before it responds to the decrease, A con trol apparatus having such characteristics is particularly suitable for electronic controls that must maintain a system within certain limits but which must not respond to minor fluctuations or the system.

For consideration or" what we believe novel our invention, attention is directed to the following specification and the claims appended ereto.

.fle improved electronic control apparatus con sists of the combination or" an amplifier that is responsive to direct input voltages constituting a signal as well as variations signal voltage a trigger circuit that has two stable operat ing conditions between which instantly transfer-s when the amplified signal voltage passes oercritical potentials. A grid controlled gaseous :3 charge connected to the trigger circuit permits current to how in a relay as long as the trigger circuit is in a particular one of its operating conditions.

The improved circuit is of particular value wen employed with photoelectric cells in apparatus for determining a change in the light trans= mission or an optical path as the transmission is varied by a quantity to he controlled. The light transmission of the optical path might Toe decreased by haze or smoke in a combustion control, by change of color or depth of color in certain industrial processes, by change of reflectance of a mirror included in the path as the mirror is clouded or obscured as dew point determining apparatus, or any similar mechanism.

The improved electronic control apparatus is schematically illustrated in the accompanying drawings.

the drawings:

Figure l is a schematic diagram of the ampli+ fier portion of the improved control circuit.

Figure 11 is a schematic diagram of the trigger circuit portion of the improved control,

Figure HI is a schematic diagram of a suitable power source for the amplifier and trigger cir ill These specific figures and the accompanying description are intended merely to illustrate the invention but not to impose limitations on the claims.

Figures l, H and ill show a circuit diagram of an electronic amplifier constructed according to the invention. The amplifier consists of an electronicallv regulated power supply (Figure iii), an impedance matching stage to connect photocells to an amplifier stage and the amplifier (Figure l) and a trigger stage for converting continu ous changes in amplifier stage output voltage into a substantially on and off signal and a thyratron providing current amplification between the triggering stage and a relay (Figure II).

Referring to Figure In power leads ti and 32 supply power to regulating transiormer it that has its secondary connected to leads ill and 853. A filament transformer ill connected to the leads l9 and fill supplies regulated filament voltage to a light source it and to the electronic tubes in-= cluded in the power supply and the amplifier. The regulating transformer it through the leads 19 and to also supplies regulated power to a plate transformer 82 having a high voltage secondary winding connected to the plates of a therm= ionic rectifier tube 8:1 and having its center tap connected to a lead constituting the negative return oi the circuit. The filament of the rectifier tube St is energized from a filament winding 86 while a lead 637 connected to the center tap of the filament winding applies high voltage to a filter condenser t8 connected between the nega= live return lead and the high voltage lead all.

The rectified voltage between the leads B5 and is subject to fluctuation with changes in the voltage between the leads 3i and 32 because of imperfections in the regulating transformer it. The variations in voltage are minimized or com pletelyeliminated by passing the current to be used by the amplifier through a regulating circuit including a regulating tube 99 which provides a voltage drop that is continuously adjusted to provide constant voltage to the amplifier. Current from the high voltage lead 6'! enters at the.

plate of the regulating tube to, passes through the tube to its filament to and then through a filament winding ill of the transformer 82 and out through the center tap of the winding 98 and a lead 92 to the positive side of a voltage divider consisting of a series of resistances connected hetween the regulated high voltage line 92 and the negative return line 85.

The voltage applied to a grid 94 of the regulat tube 539 controls the voltage drop through the tube 89. The control voltage is obtained, after amplification, connected between the regulated high voltage line 92 and the return line 85. A potentiometer 95 forms two arms-one side-of the voltage sensitive bridge. A resistor 96 and a gas discharge tube 9! form the other two arms-the other side-of .the voltage sensitive bridge. The resistor 98 is connected to the high voltage lead 92 while the gas discharge tube ill has one terminal connected to the negative return lead 85;.

The gas discharge tube el, which may be a neon glow tube, has the peculiar property that the voltage drop across it remains substantially constant for considerable variations in current flow.

. The other three arms of the bridge are linear re= sistances and the net result is that the bridge from a voltage sensitive bridge may be balanced for any desired input voltage by ad ustment of the potentiometer 9'5 and that any change in voltage applied to the bridge will upset the balance of the bridge. An amplifier tube 98 has its cathode so connected to the junction between the resistor s6 and the gas tube 97; and its control grid lot connected to the adjustable arm of the potentiometer 95. The grid-cathode cir cuit oi the amplifier tube 98 thus constitutes the detector for the voltage sensitive bridge. The plate mi of the amplifier tube to is connected through a resistor ill? to the regulated high voltage lead 92. The grid cc of the regulator tube 89 is connected to the plate of the amp ifier tube 98 and is thus controlled according to the balance of the voltage sensitive bridge. The voltage sta= bility of the gas discharge tube 9? and the substantially constant cathode temperature of the amplifier tube 98 resulting from the regulation afforded by the regulating transformer is assures an extremely constant regulated voltage across the voltage divider 93.

Because the amplifier must respond to very slow changes in potential it is necessary that the signal circuit through the amplifier be direct coupled in contrast to the resistance-capacity or inductive coupling arrangements ordinarily employed. Such an amplifier requires relatively high negative voltages for certain of its elements and these voltages are secured by grounding an interm diate point I02 of the voltage divider 93 rather than the negative end of the divider as is ordinarily done.

A pair of photocells 26 and 21 are arranged to view the light source 20. The optical path between the light source and the first photocell is shown partially obstructed by a mask 28 that is representative of variable transmission produced by the system to be controlled. The path from the light source 20 to the other photocell 2'! is partially obstructed by an adjustable mask 29 that is adjusted to secure equal light transmission through the two paths for a particular initial condition of the controlled system.

Photocells 26 and 21 are connected in series'between a lead I04 connected to a point Hi5 of the voltage divider 93 giving approximately 50 volts positive with respect toground and the negative return lead 85 w ch is maintained .lapproxipoint i833 of mately 45 volts negative with respect to ground.

The junction between the photocells is connected through a resistor I06 to the control grid ll of an amplifier tube ")8. The amplifier tube I08 is a triode-connected pentode with its screen grid, its suppressor grid and its plate connected together and to the lead it. Its cathode we is connected through resistors H0 and ill to the negative return line 85. The grid it! is cond nectedthrough a grid leal; lit to the junction between the resistors lit and iii. rangement any unbalance the light distribu= tion between the photocells 2e and ill causes then voltage drops to change accorly and thus shift the potential of their coon connection. This change in potential is applied through the resistor tilt to the grid till of the amplifier tube tilt to change the conductance of the tube.

Because of the resistance in the cathode cir= cult of the tube the cathode potential varies in the same direction and in nearly the same amount as the change in grid potential. Because of the change in potential at the junction oi t re resistors tit and iii. to which the end oi the grid leak M2 is attached, the circuit presents an unusually high impedance electric cells are concerned. The output pedance oi the stage, however, is approately the resistance of the resistors lit and iii which is very low compared to the pedance oi the photocells. A voltage dividing connected between the cathode its and the negative return lead serves to transmit the changes in cathode potential of the amplifier tube ice as well as allow adjustment of the absolute potential oi the output of the tube 888.

The movable point of the potentiometer M3 is connected through a lead lid to the control grid lie of an amplifier tube Md. The amplifier tube He has its cathode ill connected directly to a lead lie that is connected to the grounded the voltage divider and serves as the ground or zero voltage connection. Anode lls and screen and suppressor grids E29 and wt of the tube He are connected through a plate resistor i2? and a lead i223 9553 at a point having a potential of about $5 volts. The anode lie of the amplifier tube tilt is also connected through resistors i125, i and voltage divider ms to the negative return lead 5. The adjustable connection of the voltage divider 52% serves as the output of this amplifier stage.

The signal voltage at the output of the amplifier stage including the tube H8 is of the so called single-ended" variety which must be converted into apush-pull signal to operate the succeeding stages of the amplifier. This is done by connecting the adjustable point of the voltage divider see to the control grid E2? of a pent'ode amplifier tube 92c whose cathode 112s in common with the cathode lei) of an amplifier tube tilt is connected through a cathode resistor 32 to the grounded lead MB. A control grid its oi the tube i3! is connected directly to the grounded lead M3. Screen grids E36 and 535 of the amplifier tubes E28 and it i, respectively, are connected together and to the lead ltd so that the screen grids are held at a potential of approximately 5t volts positive with respect to ground. The anode ltd of the amplifier tube are is connected through a resistor Hill to a lead idil connected to the regulated high voltage output lead 92. Similarly the anode I39 or the amplifier tube ltl is connected through a resistor Mt to the lead H8. The amplifier tubes I28 and let convert the incoming signal into two amplified signal voltages having similar amplitudes and opposite phases. This results because a signal voltage applied to the grid I21 of the amplifier tube in is amplified and appears in reversed phase at its anode ltd, while the cathode I29 of the tube tends to follow the changes in grid voltage because of the presence of the cathode resistor H32. The changes in the potential of the cathode I29 are also applied In this ar= as far as the photo= potentiometer i it to the voltage divider to the cathode I30 of the tube |3l whose grid I33 is grounded so that signal voltage is applied to the cathode of the grid cathode circuit of the tube I 3|. If the signal voltages are applied to a grid the signal voltages at the plate of a tube are 180 out of phase with the grid signal, while if the signal voltage is applied to the cathode of a tube, the amplified plate voltage is in phase with the signal voltage. Because of this phase inversion, the output voltage of the tube I28 is 180 out of phase with the output voltage of the tube I3l.

The improved circuit is arranged so that it is very sensitive, but that once having responded to an increase (or decrease) in signal it will not respond to a decrease (or increase) until a substantial change in signal has occurred. This is accomplished by feeding the output voltage of the push-pull amplifier stage that includes the amplifier tubes in and BI into a trigger circuit that has two stable operating conditions, and which shifts suddenly from one condition to the other when certain threshold input voltages are reached. This trigger stage comprises two amplifier tubes Mi and 2 (Figure II) whose cathodes M3 and M4 are connected together and to the lead ltd. Control grids M5 and t lt are connected to the anodes H6 and H39 of the tubes its and iii! respectively, and thus are subjected to the slowly changing voltages corresponding to the changes in potential of the common con nection between the photocells 2d and 22 An-- odes Hit and M8 of the tubes Ml and M2 are connected through resistors i 39 and it'd, re= spectively, to the lead i365 connected to the regulated high voltage output lead 92. Screen grids 352 and 553 are connected through resistors and let, respectively, to the lead 136. Suppressor grid we of the amplifier tube Mi is con nectecl through a resistor 557 to the negative return lead and also through a resistor tilt to the anode i i-ti of the amplifier tube 6 In like manner a suppressor grid it!) of the tube M12 is connected through a resistor lei to the negative lead 35 and is connected through a resistor it? to the anode Ml of the amplifier tube t ll. The suppressor grid to anode resistors G59 and M32 are paralleled by low capacitance condensers and ltd to reduce their high frequency imped= ance. Because of the control of the plate cur rent exercised by the suppressor grid, this cir-= cuit has the characteristic of having two stable conditions, the first with the tube t ll conducting, the second with the tube M2 conducting. The interconnection of the suppressor grids and anodes acts to cut on: plate current in one tube as soon as plate current flows in the other tube. The low capacitance condensers H63 and Mid serve to accelerate the change from one conducting condition to the other.

In operation, if the grid M5 is at approximately the same potential as the cathode M3 of the amplifier tube Ni and at the same time the grid i it is highly negative with respect to the cathode t lt, the tube Mi will draw plate current so that its plate potential is reduced. The reduction in plate potential is transmitted through the resistor i652 to the suppressor grid tilt of the tube M2 so that plate current in the tube M2 is cut oil even though there may be current flow between its cathode Hi4 and its screen grid i533. As the potential of the grid MB of the non-conducting tube M2 is raised, screen current will flow but there will be no change in the anode circuit until the potential of the control grid M5 oi the tube I4! is reduced to cut ofl the flow of current through the cathode I43 of the tube I4 I. Cutting oil the flow of cathode current cuts oiT current flow from both screen and anode so that the potential of the anode I41 rises and, in rising, raises the potential of the suppressor grid I60 0! the tube I42 to permit current flow past the screen grid I53 to the anode I48.

In this arrangement, plate current is cut off in both tubes if both grids are simultaneously biased negative with respect to the cathodes, screen current can flow in both tubes if both grids are simultaneously at or just slightly below cathode potential, but in the latter condition because of the suppressor grid potentials only one of the tubes can carry anode current. Inasmuch as the amplifier is adjusted so that the trigger tube grids do not go highly negative simultaneously the condition of simultaneous anode current cutoff does not occur. Rather, one grid goes positive as the other goes negative so as to produce corresponding changes in screen current and, because of the cross connection between anodes and suppressor grids, sudden transfer of conduction from one anode to the other. The selected operating potentials and the circuit constants are such that an appreciable change in signal is required to effect a subsequent transfer of plate current from one anode to the other following the preceding transfer to that particular operating condition. The effect of this circuit is to convert slowly changing potentials from the output of the previous amplifier stage into suddenly changing potentials suitable for relay actuation.

A grid controlled gaseous discharge tube il -c5 has its grid ltd connected through a resistor it? to the anode l ll of the trigger circuit tube i ll. The discharge tube cathode W8 is connected through a resistor ltd to the anode i 8 of the other trigger circuit tube 5 32. A power circuit controlled by the gaseous discharge tube or thyratron may be traced from its cathode iLltl through a lead ill], a high voltage secondary ill of a transformer H2, 2. lead a current coil 34 of a relay contacts lid of a thermal time delay tube H5 and a lead i'lh connected to the anode ill of the discharge tube Hit. The pr mary oi the transformer ll? is connected directly to the power leads ill and 32 rather than to the regulating transformer it? because this circuit is not adversely affected by changes in line voltage. As long as the control grid ltd is positive with respect to the cathode Hill, the gaseous discharge tube W5 permits current to flow through the control relay during those half cycles of the alternating current supply when the plate of the discharge tube is positive with respect to its cathode. On the alternate half cycles the gaseous discharge tube M55 acts as an open circuit and no current is permitted to how. As soon as the trigger circuit is driven to its other condition by a change in input voltage to the amplifier, the grid tilt is driven negative with respect to the cathode the so that the tube does not pass current on either half cycle.

The thermal time delay tube W5 is included in the circuit to prevent current flow through the gaseous discharge tube i65 until its cathode has reached normal operating temperature.

This amplifier circuit because of its freedom from extraneous voltage changes provides a highly satisfactory means for converting extremely small changes in light intensity into currents suificiently large to produce reliable non-chattering operation of a control relay. Its sensitivity when used, tor example, in a dew point recorder is suficient to reliably detect tracw of condensation which are practically invisible to the naked eye and its adjustments are sufiiciently stable to permit c0n= tinuous omration with a minimum oi attention.

Having described the invention, we cl:

1. In an apparatus for translating small changes in light transmission into relay operat= ing current, in combination, a pair of series connected photocells that are exposed through dit= ferent light transmission paths to a common light source, a multistage direct coupled amplifier for converting changes in potential of the common connection between the photocells-intc amplified potentials having push-pull characteristics, a, tri ger circuit connected to the amplifier output, said trigger circuit having two stable operating con ditions between which it instantly transfers when the amplifier output potential passes certain val= ues, and a relay circuit that is operated while the trigger circuit is in a particular one or its two stable conditions,

2. In. an. apparatus for translating small changes in light transmission into relay operating current, in combination, a pair of series connected photocells that are exposed through different light transmission paths to a common light source, a direct coupled amplifier having a cathode loaded input stage connected to the common connection between the photocells, said amplifier terminating in parallel output stages, a symmetrically connected trigger circuit connected to the amplifier output the trigger circuit having two stable operating conditions between which it t instantly transfers at prede min d m output potentials, and a relay circuit couples a grid controlled gaseous discharge tube clon= nected to the trigger circuit for delivering relay operating current while the trigger circuit is in a particular one oi its operating conditions.

3. In. an apparatus for translating ll changes in light transmission into relay opereting current, in combination, a pair of photocells one of which is exposed to the changes in light, said photocells being connected in series between a source of negative potential and a source oi positlve potential, a cathode loaded plifier having a grid connected to the common connection hetween the photocells, a variable resistance el ment, said amplifier serving to produce potentie changes across the resistance element correspond ing to changes in potential of the common photo cell connection, a direct coupled amplifier con nected to the variable resistance element for in creasing the magnitude of. the potential changes, a phase inverting amplifier connected in parallel with at least a portion of the direct coupled ancplifier whereby the output of the amplifier con= sists of two equal and oppositely phased voltages. a trigger circuit comprising a pair of amplifier tubes cross connected to provide two stable op erating conditionsthat are alternatively selected according to the output potential of the direct coupled amplifier, and a relay circuit connected to the trigger circuit and arranged to operate a relay when the trigger circuit is in. a particular one of its two stable operating conditions.

some J. m. RALPH =91 

